1. Field of the Invention
This invention relates to a method for preparing a silicone rubber base useful as a source of silicone rubber for use in electric, automotive and business machine applications and a silicone rubber composition comprising the silicone rubber base. More particularly, it relates to a method for preparing a high fatigue endurance liquid silicone rubber composition within a relatively short time which maintains a stable viscosity during storage and cures into an elastomeric product having improved fatigue endurance.
2. Prior Art
In the current silicone rubber market, attention is paid to liquid silicone rubber compositions which are smoothly flowing, readily applicable through injection molding machines and automatically processable. Among others, many methods have been developed for the preparation of addition curing type liquid silicone rubber compositions of the versatile product formulation based on alkenyl group-containing organopolysiloxane. These methods are designed to comply with a line for the manufacture of various types of products. A liquid silicone rubber base is utilized as a common intermediate, and a particular silicone rubber composition is prepared by blending the base with appropriate curing agents and other ingredients.
The silicone rubber base is prepared by various methods which are either batchwise or continuous although the continuous methods suffer from more problems. For example, JP-B 47664/1991 and JP-A 130344/1986 propose to mix predetermined amounts of an alkenyl group-containing organopolysiloxane as a base component and a powdery silica filler. These methods take a long time for the manufacture of a desired composition because an initial mixing time of more than 1 hour and a heat treating time of 1 to 30 hours above 140.degree. C. are generally required for a batchwise formulation. For the manufacture of silicone rubber base using a large scale manufacturing installation, about 30 hours is sometimes necessary when both the treatment times are combined. On the other hand, the continuous methods do not insure a sufficient treatment time because the residence time within the kneader is very short. Then the liquid silicone rubber base thickens and loses shelf stability. For the cured product (silicone rubber) obtained from such a liquid silicone rubber base, there also occur losses of basic rubbery properties such as elongation and tensile strength and a loss of fatigue endurance due to insufficient dispersion of the filler.
Solutions to this problem have been proposed. JP-A 32909/1994 corresponding to EP 568891 A discloses a method for continuously preparing a liquid silicone rubber while providing an average residence time of more than 15 minutes. JP-A 102007/1990 corresponding to U.S. Pat. No. 5,198,171 discloses a method for continuously preparing a base compound comprising a polyorganosiloxane having a higher viscosity (raw rubber) as a main component in admixture with a reinforcing silica filler and various mixing assistants (i.e., so-called "wetter" or "wetting agent" which can improve affinity or wettability and dispersibility between an organopolysiloxane and silica fillers). Since simply mixing the components in a continuous kneader takes a long time until a uniform mix is obtained, this method involves previously uniformly dispersing the polyorganosiloxane and the silica filler in a high speed mechanical shearing mixer to form a free flowing powder and continuously feeding the powder into a twin screw extruder, thereby preparing a silicone compound within a short time.
The above method uses organosilanes (e.g., diphenylsilane diol, dimethylsilane diol, dihydroxypolydimethylsiloxane and dimethoxypolydimethylsiloxane) or low viscosity polysiloxanes as the mixing assistant. With only these mixing assistants used, it was difficult to impart satisfactory flow and good shelf stability to the liquid silicone rubber base.
When hexamethyldisilazane is used instead of the silicone oil, the resulting composition is improved in flow, but becomes deteriorated in cured properties, loses viscosity stability during storage, and fails to provide fatigue endurance.